) causes intense irritation by burrowing under the scales of the leg of a chicken, producing at first a whitish film and then mounds of pale yellow debris firmly attached to the leg. In severe cases the crusts can cut off the circulation in the leg and gangrene can set in.
It is easy to see the early stages on black-legged birds but not on lighter-coloured legs. There is a musty smell (like mice) on the legs if the mites are affecting them, so it is always worth a sniff when handling the birds.
The mites are acquired by contact with other birds, so could be transferred via wild birds or from a show. Control is achieved after infestation by dunking the legs once a week, for three weeks, in a widemouthed jar of surgical spirit, or putting a thick layer of petroleum jelly on the legs, which cuts off the air supply to the mites, but is rather messy. Treating birds after a show is sensible and preventing wild birds from accessing your poultry is advisable.
Old-fashioned remedies of diesel or creosote should not be used as these are harmful to the hens.
Scales, like feathers, are moulted once a year, so after the crusts have fallen off (the flesh is raw beneath so do not pull the crusts off), heavily infested legs may take a year to look normal again. Sometimes the mites will affect the face causing crusting and irritation.
Until the scales have moulted and the legs look normal again, the bird should not be shown and judges should indeed pass over birds thus affected.